The present invention relates to pumps, and more particularly, to a rotary vane pumps having hydraulic vane actuation.
Rotary pumps are widely used in industry. A conventional rotary vane pump includes a rotor assembly positioned within a rotor chamber. The rotor assembly includes a number of vanes spaced around the rotor to divide the rotor chamber into a series of discrete cavities. As the rotor assembly rotates, the vanes rotate following the wall of the rotor chamber, thereby causing the cavities to rotate around the rotor chamber. In a typical single-action rotary pump, the rotor is mounted concentrically within an eccentric rotor chamber, thereby defining a single pumping chamber. As a result of the eccentric alignment, the cavities expand and contract once during each rotation of the rotor assembly. In a double-action rotary pump, the rotor is mounted within an elliptical rotor chamber, thereby defining a pair of radially-opposed pumping chanbers. As a result, the cavities expand and contract twice during each revolution of the rotor assembly. An inlet communicates with the pumping chamber at a location where the cavities expand. Similarly, an outlet communicates with the pumping chamber at a location where the cavities contract. As each cavity expands, a partial vacuum is created which draws pumpage into the cavity through the inlet. As each cavity contracts, the pressure within the cavity increases, thereby forcing the pumpage out of the cavity through the outlet. The expansion and contraction process continues for each cavity to provide a continuous pumping action.
Sliding vane rotary pumps include generally straight vanes slidably fitted within radially extending slots formed in the rotor. As the rotor spins, centrifugal force urges the vanes out of the slots into contact with the wall of the rotor chamber. This outward force on the vanes is counteracted by a number of forces, including the viscosity of the pumpage and the friction between the vane and the vane slot. Often, the counteracting forces are large enough to cause the vanes to move slowly from or become stuck in the vane slots. This is particularly true when pumping highly viscous pumpage. In such cases, the vanes do not remain in firm contact with the wall of the rotor chamber and therefore pumpage is permitted to flow between adjacent cavities. This cause the pump to "slip," thereby losing some efficiency.
In an effort to overcome this problem, a number of methods have been developed to increase the outward force on the vanes. For example, a variety of pumps have been developed which utilize hydraulic pressure generated by the pump to increase the outward force on the vanes. In one such design, pumpage from the discharge is directed beneath the vanes to provide increased outward force on the vanes. In this design, the pump includes a flow passage that extends from the discharge outlet to the base of the vane slots. In theory, high pressure in the discharge outlet will force some of the pumpage to flow through the passage into the vane slots beneath the vanes, thereby increasing the outward force on the vanes. However, this type of design suffers from a number of disadvantages. First, this design typically provides constant outward pressure on the vanes-even when the vane should be retracting. As a result, this design typically cause increased wear. Second, the volume of pumpage that flows into the vane slots will vary depending on a number of factors including the characteristics of the pumpage (e.g. viscosity), the amount of resistance in the vane slot, and the amount of pressure developed within the discharge. As a result, the effectiveness of this type of design can vary significantly from application to application.